The present invention relates to a method for washing laundry items in a program-controlled domestic laundry-care appliance, in particular a washing machine, in which washing liquid present in a tub is heated and a wash temperature is set. The invention also relates to a program controlled domestic appliance for washing laundry items by means of a washing liquid, with a tub to hold the washing liquid and a heating unit to heat the washing liquid to a wash temperature and a programmed controller.
A method for washing laundry in a program-controlled washing machine and a corresponding program-controlled washing machine are known from DE 199 06 723 B4. A wash temperature is set as a function of a selected wash program.
A method for setting the washing liquor temperature by means of a temperature measurement in a program-controlled drum-type washing machine is also known from DE 31 26 135 C1.
The washing of laundry items from a batch of laundry in a domestic laundry-care appliance, in particular a washing machine, generally takes place in three phases. In a first phase water is let in and the laundry introduced into the drum of a washing machine for washing purposes is wet. In this first phase the laundry in the inner drum is wet with the washing liquid or washing liquor which is present in the tub of the washing machine. In a second phase heating takes place to the respective target temperature of the selected wash program. In this process a heating unit is actuated in a predominantly continuous manner. Heating in this second phase is only interrupted in certain programs or program modifications. However in most cases this results in a corresponding extension of the overall wash time. In a third phase, referred to as the post-wash phase after the target temperature has been reached, temporally limited subsequent heating takes place to some degree over a predefined hysteresis in temperature regulation. In other instances once the target temperature has been reached, subsequent heating is excluded.
Heating of the batch of laundry in the second phase and any subsequent heating in the third phase each take place indirectly by way of the washing liquor. To this end a heating element is generally disposed in the lower part of the tub. The washing liquor is heated by the heating element. When the drum moves, other laundry items come into contact with the heated washing liquor. The laundry is heated by means of the exchange between the free and bound liquor.
Since the laundry in a batch of laundry is not heated directly but indirectly by way of the washing liquor, depending on the type of textile and quantity of textile there is a temperature gradient between the interior of the batch of laundry and the free liquor surrounding it. This temperature gradient is steeper, the smaller the exchange between the free and bound liquor. A larger load quantity, a diminishing fall mechanism due to a high fill ratio and a reducing ratio of free to bound liquor also significantly increase this temperature gradient.
This means that in the case of a large load the mean temperature of the batch of laundry, in other words the “laundry temperature”, is significantly below the temperature of the washing liquor surrounding the laundry items, in other words the “washing liquor temperature”. With the short wash times that are standard for domestic appliances, this can mean that the mean laundry temperature remains below certain minimum values, for example below 50° C. for adequate bleach activation, thus having a markedly adverse effect on the wash performance.
A qualified improvement can be achieved in this context by correspondingly extending the washing time, in some instances with subsequent heating. However this in turn means a significant extension of the washing time, which the user does not want.